recently we had a discussion about individual galaxy coloration at shatters.net. So I thought it might be useful and interesting, if I report here in some more detail what I did so far for Celestia.Sci in the context of an improved galaxy rendering.

In this thread I shall only focus on my astrophysical analysis that is underlying my new code. Any programming-related aspects like improved culling etc will not be addressed here.

Before getting into "medias res", let me recall (for self consistency) the basic philosophy that is underlying my present galaxy code in Celestia: the task is to visualize ALL galaxy properties that are available in the best scientific catalogs. Not more and not less! Moreover, the rendering algorithm has to work automatically (without involving human "handicraft"), and that for MANY (10 000 -100 000 ) galaxies! In the present Celestia 1.6.x version, it is apparent that my strategy was quite successful already for about 10 000 galaxies including the entire local group and after merging data from 9 scientific catalogs! Note that this is clearly a complementary approach to e.g. adding just a few hires 2D galaxy photographs in form of add-ons.

Let me just mention in passing that I am developing in parallel a uniform, scientific scheme for colors and luminances of DSOs and stars for a somewhat later stage ...

So what's the new challenge?

Clearly: implementing individual colors for galaxies (and globulars) from the information available in galaxy/globular catalogs!!

As to galaxy colors, so far, I only distinguished between two major categories: the older elliptical and lenticular galaxies (of E/S0 type) that are more reddish and the younger spirals that are more bluish. In the code, I used a generic color profile extracted from an excellent Hubble color photo (M 74). It served as a basis for galaxy coloration.

Despite certain benefits, this state of affairs implies that two spiral galaxies are always rendered with the identical color profile! Same for ellipticals...Not quite perfect yet

Let me next address what can be done to arrive at much improved, individual galaxy/globular colors!

First, have a look at the relevant astrophysical data. Without appropriate data, there is no progress, obviously .

Since a long time, it is actually known in the literature that there is a quite marked correlation between the global B-V magnitude of a galaxy and it's Hubble type! B-V is a standard measure of the visual "blueness"!

Note that larger B-V values correspond to more reddish galaxies (i.e. ellipticals,...), while smaller B-V should be relevant for the more bluish spirals. This can be quantified as follows:

First let us switch from the classical Hubble type to the numerical Hubble Stage (T). T ranges from -6 to +11 (ignoring the two special values 90,99), thus exhibiting a finer subdivision than the simple Hubble types we use in Celestia so far. Here is the precise map to the extended Hubble types from the RC3 galaxy catalogue:

In the first plot I extract from the latest RC3 galaxy catalog with 18228 entries,
http://vizier.u-strasbg.fr/viz-bin/Vizi ... ce=VII/155,
just the individual B-V values (in the Johnson filter basis) and the corresponding Hubble Stage values T. Among the 18228 entries, there are 2998 galaxies with both B-V and Hubble stage values (-6 <= T <= 11) known. Here is a plot of the B-V value versus the Hubble stage T for those 2998 galaxies:

For each value of T you can see a substantial spread of associated B-V values, but nevertheless, a marked correlation is observed. The fat black curve is a best quadratic least-squares fit to those many data. The formula for this best fit reads

Code:

B-V_th = 0.865 - 0.0334 * T - 0.000974 * T^2

Maple does such things in 1-2 seconds...

Next, let us have a look at the spread in B-V for each given value of Hubble Stage T:
This I have investigated in the next plot, where the B-V distribution is displayed for three representative values of T:

So you can clearly see the systematic trend.

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Conclusion: The discussed correlation is certainly good enough for infering from the Hubble type a semi-quantitative B-V value for all those galaxies, where it hasn't been measured explicitly!!

By extracting the known values of B-V from the catalogs, adding them to galaxies.dsc, and using the above considerations for the missing entries, we can implement for each galaxy a realistic color shift of the generic profile as function of the B-V value!

This strategy further exploits the known fact that the relative color profiles across galaxies are quite universal!
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The coding of this new improved coloration was quite straightforward. Soon I shall display some actual examples of the more individual coloration in this thread...

Fridger

Last edited by t00fri on Thu, 03-12-09, 8:56 GMT, edited 5 times in total.

Fantastic.
Is it necessary to take into account the redshift? Or isn't it strong enough for the galaxies for which the type is known (well, the spread in BV index is so big that it probably masks any effect of this except for very distant galaxies)? I wonder.
Guillermo

Fantastic. Is it necessary to take into account the redshift? Or isn't it strong enough for the galaxies for which the type is known (well, the spread in BV index is so big that it probably masks any effect of this except for very distant galaxies)? I wonder.Guillermo

Hola Guillermo,

yeah I gather, physicists (myself included) like that sort of analysis & strategy...

Mostly z is very small for the galaxies from the Ngc/Ic catalog. For higher z values (=> e.g. SDSS,...), astrophysicists indeed tend to discuss galaxy color issues in the galaxies' rest system. I certainly agree that the z-dependence of color will by far be outweighed by the uncertainties in B-V and other sources of error.

Since we are just communicating, something entirely different:

The other day I had dinner with (the lady part of) a very well known couple of Argentinian theoretical physicists, Marcela Carena and her husband Carlos Wagner. Their setting is not unlike that of my wife and myself , and over the years we have talked a lot about "couples in physics" with them They are in their end-of-fourties and have both done their PhDs at my institute, here in Hamburg, long ago. One reason was that Carlos has ancestors from Hamburg. Marcela has a prestigous position as senior staff at Fermilab/Chicago and simultaneously as a Professor for Theoretical Particle Cosmology at Chicago University.

Now comes my question to you: During our recent joint dinner, Marcela told me that she got her Master's degree in physics at your institute in Bariloche!! Presumably the two got to know each other there?

Do you know them by chance?? They are NOT the kind of people one might easily overlook

Cheers,
Fridger

Last edited by t00fri on Thu, 03-12-09, 9:03 GMT, edited 1 time in total.

Now comes my question to you: During our recent joint dinner, Marcela told me that she got her Master's degree in physics at your institute in Bariloche!! Presumably the two got to know each other there?

Wow! I don't remember them, but I have just checked that BOTH of them got their degrees in 1985 (one year before my arrival in Bariloche). It seems that Carlos stayed there as a graduate student until 1987 (without finishing his PhD, which he completed in Hamburg). That's the year after my arrival. But I was just a young student, didn't get the chance to meet everybody.

Is it possible Carlos is in this picture of 1986, together with Abdus Salam during his visit? I don't recognize him. It's dated 1986, perhaps this is the next class, not his. And there is no girl in that picture. (I found it at the alumni webpage). I should contact them to have them send some photos for the collection...

Now comes my question to you: During our recent joint dinner, Marcela told me that she got her Master's degree in physics at your institute in Bariloche!! Presumably the two got to know each other there?

Wow! I don't remember them, but I have just checked that BOTH of them got their degrees in 1985 (one year before my arrival in Bariloche). It seems that Carlos stayed there as a graduate student until 1987 (without finishing his PhD, which he completed in Hamburg). That's the year after my arrival. But I was just a young student, didn't get the chance to meet everybody.

Is it possible Carlos is in this picture of 1986, together with Abdus Salam during his visit? I don't recognize him. It's dated 1986, perhaps this is the next class, not his. And there is no girl in that picture. (I found it at the alumni webpage). I should contact them to have them send some photos for the collection...

Guillermo

Firstly, the two wrote three publications together already in Bariloche, between May 1986 and June 1987, all published in 1987:

Secondly: Carlos' PhD thesis with a colleague of mine in Hamburg (I. Montvay) was about a completely new subject (Lattice gauge theory), hence represented a "fresh start".

Finally: I magnified that photo with Salam quite a bit, but couldn't find anyone looking like Carlos as I remember him from that time. I told Marcela that I "know" a physicist from Bariloche (You). The other physicist I knew from Bariloche (Prof. Guido Beck) has long died. He has invited my wife and me to Bariloche but we never made it (getting "stuck" in Sao Paulo for 4 months instead )...

Fridger

PS: note that in my earlier post about the two, I clarified my statement about our recent dinner: only Marcela was having dinner with me that evening. Carlos was in the US. My wife was also present , as well as a Nobel Prize winner and friend (Gerard 't Hooft).

Last edited by t00fri on Thu, 03-12-09, 9:34 GMT, edited 3 times in total.

that I am currently examining for a future implementation. Unfortunately, there is also a VERY significant (apparent) discrepancy of the Steinicke's Bmag-Vmag values with the B-V data from the renowned RC3 catalog.

Concerning this issue, I am currently in correspondence with Wolfgang Steinicke.

Let me nevertheless, show you what the Problem is.
=======================================

The resulting data, I display qickly via Maple. Here is the result, involving ALL 2667 galaxies, for which the required data entries were available in the two catalogs :

++++++++++++++++++++++++++
In this plot I displayed for each galaxy, it's B-V value from RC3 versus its Bmag-Vmag value from Steinicke's 2009 catalog.
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First off, one notes that in Steinicke's catalog all galaxies only take one of the 5 discrete values 0.6, 0.7, 0.8, 0.9, 1.0 for Bmag - Vmag. Obviously there has been too much truncation at work...

However, the most important message from this plot is this:

+++++++++++++++++++++++++++++++++
If the two data sets were compatible with each other, ALL data points in this plot would have to fluctuate around that fat black line ((B-V)_RC3 = (Bmag-Vmag)_Steinicke)! Unfortunately, the data don't do that at all! One notices many large discrepancies of more than a factor of TWO!
+++++++++++++++++++++++++++++++++

As I wrote to Wolfgang Steinicke, I don't understand his Bmag-Vmag data, UNLESS he used some highly unconventional filter characteristics for his bluer galaxies (i.e. smallish Bmag-Vmag). But I couldn't find any respective documentation...

Finally, if one replaces the "B-VT" data from RC3 by the "B-VoT" data, which include in addition the corrections for redshift and galactic & internal extinction, my qualitative conclusions remain unchanged (as expected).

let me continue by discussing first for completeness the effects of galactic & internal extinction as well as that of redshift on the above B-V color versus Hubble stage correlation. Guillermo has already been asking about the effects of the redshift on the galaxy colors that we expect to be small.

Then -- towards the end-- I'll start with the very task of galaxy coloration via B-V information!

Fortunately, the RC3 catalog offers already a column (B-V)oT with the total (B-V) values corrected for redshift as well as galactic & internal extinction. The latter effects could well be more significant.

Let's see what the RC3 data say!

So, in analogy to my first post in this thread, let us first compare the correlation of (B-V) with the Hubble Stage (T) without and with the mentioned corrections:

1) Without these corrections (as above)
=============================

Quadratic least-square-fit relation

Code:

(B-V)T_th = 0.865 - 0.0334 * T - 0.000974 * T^2

2) With these corrections===================

Quadratic least-square-fit relation

Code:

(B-V)oT_th = 0.738 - 0.0397 * T + 0.000560 * T^2

What do we observe? Firstly, the general trend and also the range of the correlation remains identical. But upon a closer look, one can notice some interesting differences:
The shape of the black best-fit curve has slightly changed from convex to concave, while the initial and final (T, B-V) values have remained almost identical. Moreover, we observe a markedly smaller fluctuation of B-V values notably for negative T (i.e. on the left)

This latter feature is also evident upon comparing the two B-V distributions for the three fixed values of the Hubble Stage T:

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That's nice, since the better corrected case also gives a sharper correlation of B-V with the galaxy morphology!
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The next step is to implement a B-V dependence into our generic color profile, such that the colors of a set of reference galaxies with small B-V ~ 0.4-0.5 (blueish) and with large B-V ~ 0.8-1 (orange/reddish) are perfectly matched within Celestia.Sci !
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As to selecting reference galaxies, one has to be VERY careful, since all too often available color photographs are artificially modified/enhanced (for PR reasons ) and color biases differ strongly among different imaging sources ! So we need to use a set of blue and orange/reddish reference galaxies that are all from the SAME source. Then the relative color effects are much more trustworthy, while absolute color rescalings are relatively harmless and rather a matter of taste and monitor display

Therefore, I mainly chose as a reference, some galaxies from SDSS-DR7, the seventh major data release of the Sloan Digital Sky Survey.

before showing you next time the final result of this galaxy coloration project in form of representative, individually colored galaxies, let me tell you a bit more about the strategy and custom tools that I have used.

Custom tools (Maple) were essential for trying out various color profiles quickly, notably in comparison to my reference SDSS set that is matched to the RC3 galaxy catalog:

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First of all, how can we parametrize complex RGB color profiles for individual galaxies most easily and intuitively, i.e matched to the actual requirements of the project?
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The trick is to compose the galaxy color profiles NOT in R,G,B color space but rather in the so-called H,S,V space. Here H=Hue, S=Saturation ("color intensity") and V=Value ("brightness").

The transformation between RGB and HSV is a standard task in image manipulation. I have also written a corresponding C++ method in Celestia long ago.

Why is color matching in the HSV basis so much simpler?

Well in first approximation, during the coloration process, one may set the Value (<=> brightness) and the Saturation (<=> "color intensity") equal to some convenient, constant value across a galaxy! Then the coloration task reduces to just parametrizing the desired Hue profile (instead of THREE profiles for the individual R,G,B colors!). After the color transition (Hue) is settled correctly, one then proceeds to tune both the two intensities (S and V) in a second step. The latter does interfere very little with the Hue settings, which makes the coloration task so much simpler and more intuitive.

The Hue is usually given in degrees within the range 0 <= H <= 360 degrees. Here is a nice circular Hue display (from The GIMP):

In this example, the Hue is located along the circle, increasing counter-clockwise. Presently we have set H=0 (white bar on circle). The actual color with V=100% and S=20% is displayed as well at the bottom.

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For Celestia (.Sci), we need to map the Hue values across a galaxy to a field, the so-called colorindex, with 256 (r,g,b) - triplet entries (i = 0..255). These triplets denote the sprite color starting from the galaxy center (i=0) and increasing towards its periphery (i = 255).
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What I needed next was a mathematical function that allows to implement intuitively -- yet precisely-- the transition of the Hue from a desired starting value (hue0) to a final one (hue1), at a particular location of the colorindex (i=i0) with a certain "transition speed" = slope.

Hence I determined a very flexible and intuitive hue-transfer function from the ansatz

Code:

hue = a - b * tanh(-i /c+slope)

involving the 4 parameters a, b, c, slope besides the colorindex i.The three parameters a, b, c were fixed in terms of hue0, hue1, i0 and slope with Maple from the 3 conditions:

As desired, the transition hue0 -> hue1 gets much sharper for increasing slope parameter. Remember that the hyperbolic tangens, tanh(x), varies between -1 and +1.

For Saturation and Value profiles in the second step, I also chose some flexible, intuitive forms. As a last convenience, I implemented the option of skipping a certain band of colors (green!) during the hue transition.

The resulting tool and the ImageTools package of Maple 13 then allowed quickly AND intuitively to try out any desired H, (and S,V) profiles in terms of a realistic RGB test image.

++++++++++++++++++++++
Notably after these considerations, I can just measure the initial and final H,S,V values of any SDSS galaxies with the GIMP "pipette" tool and Maple/C++ code do the rest.
++++++++++++++++++++++

Look e.g. at this SDSS galaxy, NGC 3344 with a measured value B-V = 0.57 according to the RC3 galaxy catalog. As you can see, it's indeed quite blue but still with an ochre/orange center ... So a "visual blueness" B-V=0.57 seems adequate.

According to the GIMP "pipette tool", the colorindex profile should start around Hue = ochre/orange ~ 37 in the galaxy center, transiting quickly (slope = 5.0, say) to Hue=blue ~ 208 in the outer regions. In addition the green band of hue (50<H<180) must be skipped during the transition!

Here is what my Maple tools display for such a configuration:

First the corresponding plots of Hue, Saturation and Value
(Click for Big!)

Note that a more realistic behaviour of S,V than a constant has also been implemented. Again the "pipette tool" provides central and peripheral values for the galaxy under consideration...

Since my modified C++ code in galaxy.cpp has all these considerations implemented as well, Here is the color result of Celestia.Sci for this configuration.

Suppose, I now wanted to see what the galaxy would look like with a somewhat redder central region (and everything else the same) I just enter hue0=17, say (instead of 37) and Maple instantaneously gives this:

A Celestia.Sci run indeed confirms the redder center:

I hope this was kind of instructive? However, the actual procedure was considerably more involved ...

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